1. Field of the Invention
The present invention relates to a method of and an apparatus for friction stir welding for joining an abutment portion by moving a rotating probe relatively along the abutment portion while pressing the probe against one surface of the abutment portion at which an end of a first cylindrical member and an end of a second cylindrical member are butted together.
2. Description of the Related Art
The friction stir welding (hereinafter referred to as “FSW” as well) is generally known, in which two workpieces are joined in solid phase by utilizing the frictional heat which is generated when a rotating probe is inserted into the workpieces (objects). When the joining is performed as described above, the joined portion of the workpiece can maintain a strength which is about 80% of that of the base material, and it is also possible to prevent crystals from becoming coarse.
If the welding is performed by means of the conventional spot welding or the conventional electron beam welding by using an aluminum material as the workpiece, for example, some excessive heat is applied to the aluminum material. Therefore, the strength may be decreased due to deterioration of the material and/or change into coarse microstructure.
In contrast, the joining by FSW is performed at about 500° C. even when a metal material such as the aluminum material having a relatively low melting point (about 600° to 660° C. in the case of the aluminum material) is used. Therefore, it is possible to perform FSW by using other materials such as magnesium, titanium, and polymer as well as the aluminum material.
As an example of the application of FSW to the aluminum material, an aluminum frame of a large member such as an electric train may be exemplified. In the case of such a large member, the thickness of the aluminum material is usually not less than 5 mm considering the strength of the joining. On the other hand, if light weight is desired as well as the improvement of strength in the case of gas turbine engines or the like, it is difficult to increase the thickness of each component. For this reason, for example, a thin plate aluminum material of about 1.2 mm thickness is used to prepare an outer frame of the gas turbine engine.
However, when an abutment portion, at which both ends of thin plate-shaped aluminum materials are butted together, is joined by means of FSW to form a cylindrical member having a relatively large diameter, it is impossible to obtain a satisfactory circularity or roundness because the aluminum material is thin.
Further, when an abutment portion, at which ends of two cylindrical members are butted together, is joined by FSW, the circumferential lengths of the respective ends may not be the same. Therefore, if the cylindrical members are joined in this state by FSW, the difference in phase appears at the final position of the joint, and for example, the shape is deformed in a wavy form, resulting in wrinkles or corrugations.
Additionally, the pressing force of 1 t to 2 t is applied to the thin plate-shaped aluminum material during the joining by FSW. Therefore, some irregularity may appear at the joined portion as a result of FSW, if the aluminum material is not retained reliably.
Japanese Laid-Open Patent Publication No. 11-226759 describes a method of joining aluminum members as shown in FIG. 9, for example. A backing member 3 of the same material as that of hollow tubes 1a, 1b, is arranged at the inside of an abutment portion 2 of the hollow tubes 1a, 1b made of aluminum. A predetermined interstice or clearance C is formed between the backing member 3 and the inner surface of the abutment portion 2.
In this arrangement, a probe tip 4a of a probe 4 is inserted into the abutment portion 2 and the backing member 3 while the probe 4 is rotated at a high speed, and the probe 4 is moved along the abutment portion 2. Accordingly, the entire circumference of the circumscribing abutment portion 2 is welded by the friction stir welding.
In the case of Japanese Laid-Open Patent Publication No. 11-226759, the backing member 3 is provided as a reinforcing member of the hollow tubes 1a, 1b, and the reinforcing member is joined to the inner surfaces of the hollow tubes 1a, 1b. However, the hollow tubes 1a, 1b, to which the backing member 3 is joined as described above, cannot be used as an outer frame of a gas turbine engine, for example. Further, the interstice C is provided between the backing member 3 and the inner surfaces of the hollow tubes 1a, 1b. Therefore, it is impossible to maintain the circularity of the abutment portion 2 especially when the hollow tubes 1a, 1b are thin and each of them has a cylindrical shape with a large diameter.